Balanced Mach-Zehnder modulator

ABSTRACT

An apparatus for modulating a beam of light with balanced push-pull mechanism. The apparatus includes a first waveguide comprising a first PN junction on a silicon-on-insulator substrate and a second waveguide comprising a second PN junction on the silicon-on-insulator substrate. The second PN junction is a replica of the first PN junction shifted with a distance. The apparatus further includes a first source electrode and a first ground electrode coupled respectively with the first PN junction and a second source electrode and a second ground electrode coupled respectively with the second PN junction. The apparatus additionally includes a third ground electrode disposed near the second PN junction at the distance away from the second ground electrode, wherein the first ground electrode, the second ground electrode, and the third ground electrode are commonly grounded to have both PN junctions subjected to a substantially same electric field varied in ground-source-ground pattern.

BACKGROUND OF THE INVENTION

The present invention relates to electro-optic modulation devices and methods. More particularly, the present invention provides an improved silicon-based Mach-Zehnder modulator and methods for balancing two Mach-Zehnder arms based on silicon photonic design platform.

Over the last few decades, the use of communication networks exploded. In the early days Internet, popular applications were limited to emails, bulletin board, and mostly informational and text-based web page surfing, and the amount of data transferred was usually relatively small. Today, Internet and mobile applications demand a huge amount of bandwidth for transferring photo, video, music, and other multimedia files. For example, a social network like Facebook processes more than 500 TB of data daily. With such high demands on data and data transfer, existing data communication systems need to be improved to address these needs.

Over the past, high data rate communication has been widely implemented via optical network, in which data signals are carried by laser light that is specifically modulated using various kinds of electro-optic modulators. Mach-Zhedner modulator is a widely used electro-optic modulator often driven by a push-pull driver (or differential driver) for relieving swing voltage requirements and power consumption. For push-pull (or differential) drive, the balance between two Mach-Zehnder phase modulation arms is very important. But the performance of conventional modulator is sensitive to unbalance of two modulation arms caused by implant mask misalignment due to the use of mirror symmetric implant mask and doping profile (i.e. ‘p-n n-p’ or ‘n-p p-n’). Therefore, an improved Mach-Zehnder modulator and methods for balancing two Mach-Zehnder modulation arms based on silicon photonics platform regardless of implant mask misalignment are desired.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to electro-optic modulation devices and methods. Various embodiments of the present invention provide a silicon-based Mach-Zhedner modulator with balanced two arms. More specifically, the invention provides a Mach-Zhedner modulator including repeated p-n or n-p doping pattern to maintain balance for two modulation arms regardless of implant mask misalignment. In certain embodiments, the invention is applied for high bandwidth optical communication, though other applications are possible.

In modern electrical interconnect systems, high-speed serial links have replaced parallel data buses, and serial link speed is rapidly increasing due to the evolution of CMOS technology. Internet bandwidth doubles almost every two years following Moore's Law. But Moore's Law is coming to an end in the next decade. Standard CMOS silicon transistors will stop scaling around 5 nm. And the internet bandwidth increasing due to process scaling will plateau. But Internet and mobile applications continuously demand a huge amount of bandwidth for transferring photo, video, music, and other multimedia files. This disclosure describes techniques and methods to improve the communication bandwidth beyond Moore's law.

Serial link performance is limited by the channel electrical bandwidth and the electronic components. In order to resolve the inter-symbol interference (ISI) problems caused by bandwidth limitations, we need to bring all electrical components as close as possible to reduce the distance or channel length among them. Stacking chips into so-called 3-D ICs promises a one-time boost in their capabilities, but it's very expensive. Another way to achieve this goal in this disclosure is to use multiple chip module technology.

In an example, an alternative method to increase the bandwidth is to move the optical devices close to electrical device. Silicon photonics is an important technology for moving optics closer to silicon. For example, electric signal can be converted to optical signal by one or more silicon photonic-based electro-optic modulation devices in which a signal-controlled element exhibiting the electro-optic effect is used to modulate a beam of light. In this patent application, we will disclose an improved electro-optic modulator utilized for high speed telecommunication.

In a specific embodiment, the present invention provides an apparatus for modulating a beam of light. The apparatus includes a first waveguide comprising a first PN junction of a first p-type region interfaced with a first n-type region on a silicon-on-insulator substrate. Additionally, the apparatus includes a second waveguide comprising a second PN junction of a second p-type region interfaced with a second n-type region on the silicon-on-insulator substrate. The second PN junction is a replica of the first PN junction shifted with a distance. The apparatus further includes a first source electrode and a first ground electrode coupled respectively with either the first p-type region or the first n-type region of the first PN junction. Furthermore, the apparatus includes a second source electrode and a second ground electrode coupled respectively with the second p-type region and the second n-type region of the second PN junction. Moreover, the apparatus includes a third ground electrode disposed near the second PN junction at the distance away from the second ground electrode. The first ground electrode, the second ground electrode, and the third ground electrode are commonly grounded to have both PN junctions subjected to a substantially same electric field varied in ground-source-ground pattern.

In another specific embodiment, the present invention provides a method for manufacturing a linear Mach-Zhedner modulator with balanced arms. The method includes providing a silicon-on-insulator substrate and forming a first linear waveguide and a second linear waveguide in the silicon-on-insulator substrate. The second linear waveguide is in parallel to the first linear waveguide with a laterally shifted distance. Additionally, the method includes forming a first PN junction in the first linear waveguide and a second PN junction in the second linear waveguide. The first PN junction and the second PN junction have a substantially identical repeated p-n p-n doing profile along one direction of the laterally shifted distance. The method further includes forming a first ground electrode and a first source electrode to couple with the first PN junction and forming a second ground electrode and a second source electrode at the laterally shifted distance away to couple with the second PN junction. Furthermore, the method includes forming a third ground electrode at another laterally shifted distance away from the second ground electrode beyond the second PN junction. The third ground electrode is commonly grounded with both the first ground electrode and the second ground electrode for imposing an electric field across both the first PN junction and the second PN junction by a substantially same ground-source-ground pattern.

In an alternative embodiment, the present invention provides a method for balancing two arms of a Mach-Zhedner modulator. The method includes providing a silicon-on-insulator substrate. Additionally, the method includes forming a first linear waveguide and a second linear waveguide in the silicon-on-insulator substrate. The second linear waveguide is a substantially same structure as the first linear waveguide shifted by a distance. The method further includes masking a first region of the first linear waveguide and a second region of the second linear waveguide with repeated symmetry. Furthermore, the method includes implanting p-type impurity to the first region and the second region. The method then includes masking a third region of the first linear waveguide and a fourth region of the second linear waveguide with repeated symmetry. Moreover, the method includes implanting n-type impurity to the third region and the fourth region such that he third region interfaces with the first region to form a first PN junction in the first linear waveguide and the fourth region interfaces with the second region to form a second PN junction in the second linear waveguide. The second PN junction is substantially identical to the first PN junction.

In a specific embodiment, the method for balancing two arms of a Mach-Zhedner modulator includes forming a first ground electrode and a first source electrode respectively coupled to first region and the third region associated with the first PN junction. The method further includes forming a second ground electrode and a second source electrode respectively coupled to the second region and the fourth region associated with the second PN junction shifted with the distance away from the first PN junction. Furthermore, the method includes forming a third ground electrode shifted with the distance away from the second ground electrode beyond the second PN junction. The third ground electrode is commonly grounded with the first ground electrode and the second ground electrode such that each PN junction is subjected to an electric field imposed with a substantially same ground-source-ground pattern.

The present invention achieves these benefits and others in the context of known memory technology. However, a further understanding of the nature and advantages of the present invention may be realized by reference to the latter portions of the specification and attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The following diagrams are merely examples, which should not unduly limit the scope of the claims herein. One of ordinary skill in the art would recognize many other variations, modifications, and alternatives. It is also understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this process and scope of the appended claims.

FIG. 1 is a simplified diagram of typical Mach-Zhedner modulators with two arms formed by mirrored p-n implant masks.

FIG. 2 is a simplified diagram showing impact of implant mask misalignment on a Mach-Zhedner modulator with mirrored symmetric doping profile.

FIG. 3 is a simplified diagram of a Mach-Zhedner modulator with repeated p-n doping profile associated with two arms according to an embodiment of the present invention.

FIG. 4 is a simplified diagram showing impact of implant mask misalignment on a Mach-Zhedner modulator with repeated symmetric doping profile according to an embodiment of the present invention.

FIG. 5 is a simplified diagram showing comparison of phase changes due to EO effect in two arms of a Mach-Zhedner modulator with mirrored symmetric doping profile according to a prior art versus a Mach-Zhedner modulator with repeated symmetric doping profile according an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to silicon photonic devices and methods. Various embodiments of the present invention provide a silicon-based Mach-Zhedner modulator with balanced two arms. More specifically, the invention provides a Mach-Zhedner modulator including repeated p-n or n-p doping pattern to maintain balance for two modulation arms regardless of implant mask misalignment. In certain embodiments, the invention is applied for high bandwidth optical communication, though other applications are possible.

The following description is presented to enable one of ordinary skill in the art to make and use the invention and to incorporate it in the context of particular applications. Various modifications, as well as a variety of uses in different applications will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to a wide range of embodiments. Thus, the present invention is not intended to be limited to the embodiments presented, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

In the following detailed description, numerous specific details are set forth in order to provide a more thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced without necessarily being limited to these specific details. In other instances, well-known structures and devices are shown in block diagram form, rather than in detail, in order to avoid obscuring the present invention.

The reader's attention is directed to all papers and documents which are filed concurrently with this specification and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. All the features disclosed in this specification, (including any accompanying claims, abstract, and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

Furthermore, any element in a claim that does not explicitly state “means for” performing a specified function, or “step for” performing a specific function, is not to be interpreted as a “means” or “step” clause as specified in 35 U.S.C. Section 112, Paragraph 6. In particular, the use of “step of” or “act of” in the Claims herein is not intended to invoke the provisions of 35 U.S.C. 112, Paragraph 6.

Please note, if used, the labels left, right, front, back, top, bottom, forward, reverse, clockwise and counter clockwise have been used for convenience purposes only and are not intended to imply any particular fixed direction. Instead, they are used to reflect relative locations and/or directions between various portions of an object.

FIG. 1 is a simplified diagram of typical Mach-Zhedner modulators with two arms formed by mirrored p-n implant masks. Typical Mach-Zhedner (MZ) modulator within silicon photonic platform includes two identical Si-based waveguides, or Arms A and B. The two Si-based waveguides can be made of Si or SiN. Each waveguide includes a PN junction with a p-type region interfacing a n-type region along the whole arm length. The p-type region and the n-type region are formed by respectively doping p- or n-type electric impurity into pre-patterned waveguide material formed on a silicon-on-insulator (SOI) substrate. In one example, the p-type region of each PN junction is coupled to an electrode G_(L) or G_(R) at common ground level (V=0) and the n-type region of one PN junction (Arm A) is coupled to an source electrode S+ at positive bias level (V>0) while the n-type region of another PN junction (Arm B) is coupled to another source electrode S− at negative bias level (V<0). The electrode material can be metal or other conductive material overlying the corresponding p-type or n-type regions of each PN junction. When two waveguide arms, Arm A and Arm B, are paired to form the MZ modulator, a central electrode Gc, also at the ground level, is added between the two source electrodes S+ and S− but free from contact either source electrode or part of two PN junctions. Gc is disposed directly on the insulator layer of the silicon-on-insulator substrate. Adding central electrode Gc makes a same GSG pattern for each PN junction, forming a GSGSG five-electrode configuration bearing both mirrored and repeated symmetries around each of the two PN junctions. Further, the two PN junctions of Arm A and Arm B are disposed geometrically in a mirrored symmetric configuration, i.e., either in a p-n n-p or n-p p-n order.

In an application of using push-pull mechanism to drive the MZ modulator, bias voltages applied between the first source electrode S+ and ground electrode G_(L) plus a central ground electrode G_(C) to generate an electric field in GSG pattern for the Arm A. Similarly, bias voltages applied between the first source electrode S− and ground electrode G_(R) plus the central ground electrode G_(C) to generate an electric field in GSG pattern for the Arm B, which is mirrored symmetric to that of Arm A. The waveguide in Arm A or Arm B is configured to couple with an optical fiber. A beam of light, coming in an optical fiber, can be split into two by a power splitter and feed into corresponding two optical fibers coupled to the two waveguides, Arm A and Arm B. Accordingly, the phases of the beam of light can be properly modulated in those two arms due to electro-optic (EO) effect under the push-pull drive mechanism.

In another example, when two modulator arms Arm A and Arm B are paired to form the MZ modulator, no central electrode Gc is used. Therefore, on top of mirrored symmetric p-n n-p or n-p p-n junction configuration for each of the Arm A and Arm B, the associated four electrodes forms a GSSG mirrored symmetric configuration for the MZ modulator. Note, the doping process for forming the PN junction of each arm is typically done by impurity implantation with a precedent patterning or masking process. Usually p-type doping is done for both PN junctions but is separately done for n-type impurity doping. In the prior art, a pair of implant masks for p-type impurity doing is placed to expose corresponding p-type regions of the two arms of the MZ modulator with a designated mirrored symmetric layout. Similarly, another pair of implant masks for n-type doping is needed to expose corresponding n-type regions in order to form the required PN junctions. However, a drawback exists with the above design, since any misalignment of either n-type or p-type implant masks over corresponding regions on Arm A and Arm B would result in asymmetric p-n doping profile between the two arms of the MZ modulator, as seen in more detail below.

FIG. 2 is a simplified diagram showing impact of implant mask misalignment on a Mach-Zhedner modulator with mirrored symmetric doping profile. To the left of the figure, a perfect alignment of implant masks is done before performing doping. That results in perfect mirror symmetric p-n n-p (or n-p p-n) doping profile so that Arm A and Arm B are identical. The MZ modulator with such pair of modulation arms is able to generate identical phase changes due to EO effect in the two arms (A and B), a desired result for utilizing push-pull drive mechanism.

However, to the right of the figure, a situation of misaligned implant masks is shown. For example, mask for p-type impurity implantation is slightly tilted and/or mask for n-type impurity implantation is slightly shifted. Any of such misalignment or imperfection of the implant masking would result in break-up of mirror symmetry of the doping profiles between Arm A and Arm B. Therefore, Arm A and Arm B are no longer identical, thereby modulation phase changes due to EO effect in Arm A and Arm B of the MZ modulator are not the same, an undesired result for utilizing push-pull drive mechanism. Since the mirrored symmetric configuration is sensitive to the p and n implant masks alignment, improvement on modulation arms configuration is needed.

FIG. 3 is a simplified diagram of a Mach-Zhedner modulator with repeated p-n doping profile associated with two arms according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown, a repeated, not mirrored, symmetric p-type and n-type implant masks are provided. When the corresponding implant masks in repeated symmetric pattern are utilized for doping p-type or n-type impurities in the two silicon-based waveguides, two PN junctions with repeated symmetry are formed. The implantation process is performed for doping p-type impurity at one time into corresponding p-type regions of both waveguides based on the p-type implant masks with repeated symmetry. Then, the implantation process is performed for doping n-type impurity at another time into corresponding n-type regions of both waveguides based on the n-type implant masks with repeated symmetry.

Additionally, to form two balanced modulator arms with repeated symmetry, the corresponding electrode setting is also required to be formed in repeated symmetric pattern. Particularly for the embodiment shown in FIG. 3, a five-electrode GSGSG configuration in a direction across the two modulator arms is utilized. As shown, at least from one direction across the two modulator arms, a first part of a common ground electrode, G_(L), is disposed to couple with the p-type (or n-type) region of a first PN junction of a first modulator arm (e.g., Arm A′) followed by a first source electrode (S+) disposed to couple with the n-type (or p-type) region the same PN junction of the same modulator arm. Further along this direction, a second part of the common ground electrode (G_(R)) and a second source electrode (S−) are disposed in repeated symmetry to couple respectively with the second PN junction of the second modulator arm (e.g., Arm B′), followed by a third part of the common ground electrode. The second part of the common electrode, namely a central ground electrode (G_(C)), of course has a spacing away from the first PN junction and similarly in a repeated symmetry the third part of the common ground electrode has a spacing away from the second PN junction. The three parts of common ground electrode are at a same grounded state (V=0) while the first source electrode and the second source electrode are biased to a voltage having a same value but opposite in positive/negative polarity. In this way, each modulation arm, Arm A′ or Arm B′, is subjected to same alternating electric field induced by the bias voltages applied via GSG electrodes in repeated symmetric configuration. Detail illustration is shown below about how this repeated symmetry in the formation of both the two PN junctions and overlying electrodes improves the balance of two arms of the MZ modulator.

FIG. 4 is a simplified diagram showing impact of implant mask misalignment on a Mach-Zhedner modulator with repeated symmetric doping profile according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown, to the left of the figure, p-type and n-type implant masks are perfectly aligned in Arm A′ and Arm B′. Of course, there will be no issue at all since the p-n doping profiles of the Arm A′ and Arm B′ are identical in this situation and the resulted phase changes the two arms of MZ modulator are also identical. Alternative, to the right of the figure, a scenario of misaligned implant masks is shown. However, since either p-type implant mask or n-type implant mask is repeatedly applied to Arm A′ and Arm B′, any misalignment or shifting relative to a central line of each arm is still repeated for both Arm A′ and Arm B′. Therefore, corresponding p-n doping profiles associated with both Arm A′ and Arm B′, although not perfect by itself, remain identical among each other. Accordingly, the modulation phase changes due to EO effect in both Arm A′ and Arm B′ of a MZ modulator are still physically identical. Thus the push-pull drive mechanism can be implemented without being affected by the manufacture imperfection.

Similarly, the electrodes coupled with the p or n regions of each PN junction can be formed with repeated symmetry. Metal or other conductive materials can be deposited or plated onto pre-patterned regions for either grounded electrode or source electrode with repeated symmetry for the two arms of the MZ modulator. Though not shown in FIG. 4, the results would be similar to the formation of PN junctions as manufacture process is much easier to achieve repeated symmetric patterns than creating mirrored symmetric patterns.

FIG. 5 is a simplified diagram showing comparison of phase changes due to EO effect in two arms of a Mach-Zhedner modulator with mirrored symmetric doping profile according to a prior art versus a Mach-Zhedner modulator with repeated symmetric doping profile according an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown, the phase changes of Mach-Zhedner modulator are substantially linear over the applied bias voltages. To the left part of the figure, phase changes of two arms of a MZ modulator with mirrored symmetric doping profile are plotted. Due to broken symmetry caused by manufacture misalignment or imperfect doping profile variations, the PN junction in one arm (Arm A) is no longer identical to that of another arm (Arm B). This results in a reduced slope of the linear plot for the phase change of Arm B.

On the other hand, to the right part of the figure, the phase changes of a MZ modulator with two arms in repeated symmetric doping profile according to an embodiment of the present invention are plotted against the applied bias voltages, keeping substantially the same linear relation for the two arms, Arm A′ and Arm B′. The repeated symmetry of the PN junctions for the two arms is not affected at all by any misalignment of corresponding p-type or n-type implant mask and resulted imperfect doping profiles within each arm, either Arm A′ or Arm B′. In other words, the PN junction in Arm A′ remains identical to that in Arm B′. When a beam of light is split into the two arms, phase changes in Arm A′ and Arm B′ are the same so that the push-pull phase modulation mechanism can be properly executed for the MZ modulator.

While the above is a full description of the specific embodiments, various modifications, alternative constructions and equivalents may be used. Therefore, the above description and illustrations should not be taken as limiting the scope of the present invention which is defined by the appended claims. 

What is claimed is:
 1. An apparatus for modulating a beam of light, the apparatus comprising: a first waveguide comprising a first PN junction of a first p-type region interfaced with a first n-type region on a silicon-on-insulator substrate; a second waveguide comprising a second PN junction of a second p-type region interfaced with a second n-type region on the silicon-on-insulator substrate, the second PN junction being a replica of the first PN junction shifted with a distance, wherein the first PN junction and the second PN junction comprise either p-n p-n or n-p n-p doping profile in repeated symmetry; a first source electrode and a first ground electrode coupled respectively with either the first p-type region or the first n-type region of the first PN junction; a second source electrode and a second ground electrode coupled respectively with the second p-type region and the second n-type region of the second PN junction; and a third ground electrode disposed near the second PN junction at the distance away from the second ground electrode, wherein the first ground electrode, the second ground electrode, and the third ground electrode are commonly grounded to have both PN junctions subjected to a substantially same electric field varied in ground-source-ground pattern.
 2. The apparatus of claim 1 wherein the first waveguide and the second waveguide are in linear shape parallel to each other.
 3. The apparatus of claim 1 wherein the first waveguide and the second waveguide are made of Si or SiN patterned within the silicon-on-insulator substrate.
 4. The apparatus of claim 1 further comprising a power splitter to split the beam of light via a first optical fiber and a second optical fiber respectively coupled into the first waveguide and the second waveguide.
 5. The apparatus of claim 4 wherein the split beam of light in the first waveguide is subjected to phase modulation driven from the electric field imposed on the first PN junction by a first bias voltage between the first source electrode and the first ground electrode, the split beam of light in the second waveguide is subjected to phase modulation driven from electric field imposed on the second PN junction by a second bias voltage between the second source electrode and the second ground electrode.
 6. The apparatus of claim 5 wherein the first bias voltage and the second bias voltage is equal in value but alternative in either positive or negative polarity.
 7. The apparatus of claim 1 wherein the first source electrode and the second source electrode are alternatively applied a positive and a negative bias voltage relative to ground.
 8. A method for manufacturing a linear Mach-Zehnder modulator with balanced arms, the method comprising: providing a silicon-on-insulator substrate; forming a first linear waveguide and a second linear waveguide in the silicon-on-insulator substrate, the second linear waveguide being in parallel to the first linear waveguide with a laterally shifted distance; forming a first PN junction in the first linear waveguide and a second PN junction in the second linear waveguide, the first PN junction and the second PN junction having a substantially identical repeated p-n p n doping profile along one direction of the laterally shifted distance; forming a first ground electrode and a first source electrode to couple with the first PN junction and forming a second ground electrode and a second source electrode at the laterally shifted distance away to couple with the second PN junction; and forming a third ground electrode at another laterally shifted distance away from the second ground electrode beyond the second PN junction, the third ground electrode being commonly grounded with both the first ground electrode and the second ground electrode for imposing an electric field across both the first PN junction and the second PN junction by a substantially same ground-source-ground pattern.
 9. The method of claim 8 wherein the first/second source electrode is coupled to either a p-type region or a n-type region of the first/second PN junction.
 10. The method of claim 8 wherein forming the first linear waveguide and the second linear waveguide comprise patterning, depositing, or doping Si or SiN within the silicon-on-insulator substrate.
 11. The method of claim 8 wherein forming the first PN junction and the second PN junction comprising implanting p-type/n-type impurity into a first partial region with repeated symmetry in both the first linear waveguide and the second linear waveguide at a first time followed by implanting n-type/p-type impurity to a second partial region next to the first partial region with repeated symmetry in both the first linear waveguide and the second linear waveguide at a second time.
 12. A method for balancing two arms of a Mach-Zehnder modulator, the method comprising: providing a silicon-on-insulator substrate; forming a first linear waveguide and a second linear waveguide in the silicon-on-insulator substrate, the second linear waveguide being a substantially same structure as the first linear waveguide shifted by a distance; masking a first region of the first linear waveguide and a second region of the second linear waveguide with repeated symmetry; implanting p-type impurity to the first region and the second region; masking a third region of the first linear waveguide and a fourth region of the second linear waveguide with repeated symmetry; implanting n-type impurity to the third region and the fourth region, wherein the third region interfaces with the first region to form a first PN junction in the first linear waveguide and the fourth region interfaces with the second region to form a second PN junction in the second linear waveguide, the second PN junction being substantially identical to the first PN junction, wherein the first PN junction and the second PN junction comprise either p-n p-n or n-p n-p doping profile in repeated symmetry; forming a first ground electrode and a first source electrode respectively coupled to first region and the third region associated with the first PN junction; forming a second ground electrode and a second source electrode respectively coupled to the second region and the fourth region associated with the second PN junction shifted with the distance away from the first PN junction; and forming a third ground electrode shifted with the distance away from the second ground electrode beyond the second PN junction, the third ground electrode being commonly grounded with the first ground electrode and the second ground electrode such that each PN junction is subjected to an electric field imposed with a substantially same ground-source-ground pattern.
 13. The method of claim 12 wherein the first linear waveguide and the second linear waveguide are made of Si or SiN patterned within the silicon-on-insulator substrate.
 14. The method of claim 12 wherein the third ground electrode is configured to provide an electric field with a ground-source-ground electrode pattern across the second PN junction substantially same as that across the first PN junction.
 15. The method of claim 14 wherein the first source electrode and the second source electrode are configured to be biased with a same voltage but opposite in polarity relative to ground. 